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How to track the Earth’s rotation with a PlayStation Move controller

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Why should Microsoft's Kinect sensor have all the fun? One of the other motion-sensing game controllers, Sony's bulb-headed PlayStation Move, has been hacked to detect the Earth's rotation, find geographic north, and determine latitude.

Gyroscopes used to be the gadget-of-choice in sophisticated navigation systems for things like aircraft, nuclear submarines and rockets. These days, you're likely to find a tiny mechanical gyroscope in your GPS, iPhone 4 or, of course, PlayStation Move controller.

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Gyroscopes measure the rate of rotation, but not with respect to the ground. They do it with respect to an inertial frame of reference that's aligned with the stars. Seeing as the Earth's rotation also follows this same "celestial sphere", a stationary gyroscope can sense that rotation.

But while sophisticated devices for tasks like underground drilling and deep-sea navigation are perfectly tuned to detect that motion, a group of French tinkerers called PABR Technologies wanted to see if they could get the same results on a cheap, consumer-grade gyroscope. As the PlayStation Move can easily send its data over Bluetooth, PABR chose Sony's motion-sensing controller for the task.

The PlayStation Move is one of the most accurate motion controllers about and is definitely more sensitive to movement than Nintendo's Wii Remote, but it's optimised for rates of rotation that are suitable for shooting bad guys, casting magic spells and stacking colourful boxes -- not tracking the planet's rotation. While detecting smaller amounts of rotation it suffers from noise and a drifting bias.

There's a good reason for that. If the PlayStation Move gyros were too accurate, the International Traffic in Arms Regulators might be forced to categorise the controller as a missile component. "This is why we can't have nice motion tracking," says PABR.

So by strapping the Move to a turntable, which is moving at a constant 45 RPM, it "shifts the signal of interest to a region of the spectrum where the noise can be filtered more easily." Also, magnetic fields can influence gyro measurements, so PABR had to flank the Move with two black Helmholtz Coils; a pair of devices that produce a region of nearly uniform magnetic field.

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Finally, it's done. The Linux laptop receives the raw data over Bluetooth, performs some mathematical wizardry, and spits out the Earth's rotation rate. It's not perfect, says PABR, but "with a few improvements our approach will allow millions of individuals to confirm Earth's rotation for themselves at negligible cost and effort."

"Without advanced technical skills, and using only household items, [this is] a democratisation of the Copernican revolution made possible by the consumer electronic industry," the modding team claims.